1. Introduction
Carbon dioxide (CO
2) plays a crucial role in the process of photosynthesis, which is essential for the growth of plants and, consequently, vital for the existence of animal life on Earth [
1]. Additionally, CO
2 serves as the primary greenhouse gas (GHG). GHGs absorb and emit infrared radiation from the Sun, warming the Earth’s surface and the lower levels of the atmosphere [
2]. Naturally occurring, it historically constituted roughly 300 parts per million (ppm) or 0.03% of the Earth’s atmosphere. During ice ages, concentrations remained at approximately 200 ppm, while in interglacial periods, they declined slightly below 300 ppm. Scientists largely attribute this surge in CO
2 concentration to human activities, and it is recognized as the primary contributor to global warming [
3].
The influence of human activities is believed to have led to an increase of about 1.0 °C in global temperatures above pre-industrial levels, with a probable range of 0.8 °C to 1.2 °C. If the current trend persists, the Intergovernmental Panel on Climate Change (IPCC) predicts that global warming is likely to reach 1.5 °C sometime between 2030 and 2052 [
4].
The majority of CO
2 emission reduction models endorsed by the IPCC necessitate substantial reliance on CCUS. As per the IPCC, the adoption of Carbon Capture, Utilization, and Storage (CCUS) is imperative to keep the atmospheric CO
2 concentration below 450 ppm by the year 2100 [
5].
Det Norske Veritas (DNV) reports that existing CCUS facilities worldwide can capture 41 MtCO
2/yr, just 0.1% of total CO
2 emissions [
6]. The global project pipeline now represents over 400 MtCO
2/yr capture capacity expected to be online by 2030 [
7]. However, the average of IPCC’s global net CO
2 emissions scenarios anticipates that the energy sector alone must hold a sequestration capacity of 12 GtCO
2/yr by 2050. Consequently, carbon capture technologies are crucial for the decarbonization process, and achieving zero net emissions quickly might be unattainable without their contribution.
It is important to acknowledge that while CO
2 is a tradable commodity, it lacks an established market. Additionally, Carbon Capture and Utilization (CCU) serves as a supplementary approach rather than a substitute for Carbon Capture and Storage (CCS), according to the IEA [
8]. The impact of CCU on international CO
2 emissions reduction is expected to be minimal, with approximately 0.2 GtCO
2/yr by 2050, and it would not rival CCS due to its significantly higher CO
2 capture capacity, which is expected to reach 7.8 GtCO
2/yr by 2050 [
9]. In the Net Zero Scenario published by the IEA [
10], almost 90% of CO
2 sourcing from Bio-Energy with Carbon Capture (BECC) and Direct Air Capture (DAC) is sequestered, with less than 15% used as feedstock for other products (for example, CO
2-derived fuels).
CO
2 transportation by ship is anticipated to assume a significant role in the initial phases of CCS development, especially for modest capacities and/or for long-distance transportation [
11]. The Global CCS Institute [
12] states that CCUS technological advances facilitate both the capture and transportation of CO
2 within maritime operations. Firstly, vessels fitted with these technologies have the capability to capture CO
2 emissions generated through the combustion of hydrocarbon-based fuels onboard. This process involves the utilization of scrubbers, which are presently employed for purifying emissions from exhaust gases and can be adapted for CO
2 capture. These technologies would enable shipping companies to extract substantial amounts of CO
2 from their exhaust emissions. Secondly, ships can convey the captured CO
2 to its designated delivery location. Providers of technology have devised safe methods for CO
2 storage during ship transportation, thus ensuring that the pressure and temperature are appropriate, which is comparable to storage solutions for substances like liquid petroleum gas (LPG). These similarities with LPG are also applicable to port infrastructures, including facilities for CO
2 liquefaction, temporary storage, and loading/unloading [
13]. As noted by Xing et al. [
14], shipowners have various materials to choose from for CO
2 tanks and can enhance cargo hold volume by employing a single sizable tank or multiple smaller tanks. CCS technology integration in maritime applications is at early development stages, and its future sustainability depends on a combination of considerable scientific advances and supportive regulatory measures.
The transportation phase in the carbon value chain acts as the bridge between emission sources and storage locations. In addition to pipelines, CO2 transportation by ship offers a versatile and expandable CCS platform capable of accommodating upcoming initiatives. Vessels are particularly advantageous for handling CO2 sources that may not warrant the installation of a dedicated pipeline due to its size or capacity.
Gas transported at pressures near atmospheric levels requires very large facilities due to its expansive volume. However, by compressing the gas, it occupies less space and can be transported through pipelines. Further reduction in volume can be achieved through different processes [
15].
CO
2 can exist in either a gaseous or solid state depending on the temperature at atmospheric pressure. Only reducing the temperature at sea level pressure will not result in the liquefaction of CO
2. CO
2 as a liquid can only be achieved through a relatively low temperature combined with a pressure level significantly higher than atmospheric pressure [
16], as shown in
Figure 1. CO
2 can be liquefied at different pressures within the range spanning from the triple point (5.18 bar, −56.6 °C) to the critical point (83.8 bar, 31.1 °C). Compression of CO
2 can induce a supercritical state characterized by increased density, thereby avoiding bi-phase flow conditions when subjected to pressures beyond its critical pressure and temperature [
17].
At this moment, CO
2 can be transported in three distinct forms to either offshore subterranean storage facilities or onshore reception facilities [
19]:
Gaseous transportation: CO2 is carried via pipeline, using intermediate boosters, after being compressed to 35 bar;
Liquid transportation: Pipeline or ships are used to convey compressed CO2;
Supercritical transportation: Pipelines are used to convey CO2 that has been compressed to 250 bar.
CO
2 transport by ship is a mature technology founded on the shipping expertise in the alimentary sectors. It has been used on a small scale for more than 30 years, with just 3 MtCO
2 per year. In light of the fact that CO
2 shipping may be more cost-effective than building new extended-range pipelines or adapting gas pipelines at current loading facilities and discharge platforms, CO
2 shipping is now taken into consideration for large-scale CO
2 transport [
20].
The most obvious option for ship transportation is liquefied CO
2, although ships that transport compressed, gas phase CO
2 have also been proposed. The transportation of compressed CO
2 is comparable to the transportation of CO
2 via pipelines. As a result, the circumstances of transportation will be similar to those of pipelines, but more flexible and straightforward to inspect. The pressure should be above 75 bar, and the temperature should be about 25 °C as a supercritical liquid. Shipping companies have studied the concept of compressed CO
2 on board, but it has not been proven, and there are no international standards for CO
2 transport of this kind [
21].
The majority of the literature suggests CO
2 to be transported as a liquid in conditions close to the triple point for the advantages of increased density and lower storage expenses [
22]. Other research, however, points to a higher liquefaction pressure as a means of achieving greater energy efficiency. There is therefore no ideal liquefaction pressure that applies to all circumstances; rather, it should be determined based on the needs of each individual as well as the larger chain and project characteristics [
23].
The International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) is applicable to new gas carriers that are constructed after 1986. Since this year, all new ships must comply with the IGC Code, as required by the Safety of Life at Sea Convention (SOLAS) revisions [
24]. According to Kokubun et al. [
25], due to the physical characteristics of CO
2—notably, the vapor liquid equilibrium properties—the design of a liquid CO
2 (LCO
2) tank is remarkably similar to intermediate-pressure LPG onboard storage systems. Different international standards, such as the IGC Code, as well as those of classification societies like Lloyd’s Register (LR), Det Norske Veritas (DNV), and Bureau Veritas (BV), govern the well-established design process for LPG cargo tanks.
As previously stated, the insufficient number of LCO
2 carriers precludes a meaningful comparison. Because of this, it is thought that similar ships might serve as a useful foundation for comparisons pertaining to energy and techno-energetics. Similarities between LCO
2 carriers and LPG carriers (pressurized or semi-pressurized) for LCO
2 transportation can be found. Analogously, similarities can be found between Compressed Natural Gas (CNG) carriers and carriers for CO
2 transportation as a compressed gas. Nevertheless, the world fleet now consists of just one CNG ship [
26], and CO
2 transportation as a gas has not yet been developed as a workable alternative.
This article aims to determine the most advantageous pressure range for liquid CO
2 transportation in vessels covering the liquefaction of CO
2 prior to its loading on board and during its transportation by ship. To this end, a techno-energetic study comprising a series of indicators is conducted. To achieve this, two different tank configurations (described in
Section 2), a common model ship, and a specific range of pressures (specified in
Table 1) will be considered. The pressure range studied and the CO
2 conditions as a saturated liquid are listed in
Table 1.
As it will be explained in subsequent sections, the pressures considered in this table are segmented into two storage arrangements: bilobe tanks and vertical cylinders. The first one, the bilobe tank, as it will be detailed in
Section 2.1, is quite common in LPG ships; the second one is proposed for higher-pressure storage of CO
2, as the high pressures would require excessive thickness in the walls of the bilobe tanks for their fabrication. Some of the pressures listed in
Table 1 are shared between the two studied storage arrangements.
3. Results and Discussion
Figure 4 shows the specific thermomechanical exergy for the different alternatives studied, regardless of the packing circumstances. There is more exergy at low pressures and temperatures than there is at higher pressures and temperatures, considering the restricted dead state defined in
Section 2. This suggests that as the pressure rises, the expected energy required to move CO
2 from a restricted dead state to the saturated liquid state at the corresponding pressure decreases. Regarding Case No. 1 (6 bar, −53 °C), the exergy is almost 6.5% higher than Case No. 9 (45 bar, 10 °C).
Figure 4 shows that from the exergetic point of view, high pressures for liquefaction are more advantageous than the combination of low pressures and temperatures, close to the triple point. The evaluation indicates that the procedures involved in producing liquid CO
2 at 10 bar are anticipated to demand comparatively more energy than the alternatives at higher pressures. An efficient use of energy at this juncture is crucial, given that it carries an associated cost that will have ramifications throughout the entire operational lifespan of the logistical chain, which may extend over a 30-year period easily.
Different characteristic masses are presented in
Figure 5 for bilobe tanks and in
Figure 6 for cylindrical tanks. The maximum mass for storing CO
2 in both bilobe and cylindrical tanks, based on tank strength calculations, is represented in dark blue and referred to as “maximum CO
2”. The mass of tanks is labeled “Steel”. In all cases considered for bilobe tanks, the maximum CO
2 value is higher than “Alkaid’s” sum of LPG cargo and tanks. As defined in
Section 2.2 and Equation (2), to ensure a consistent comparison, the overall mass balance must remain constant relative to “Alkaid”. Hence, any surplus mass compared to the model vessel is identified and discounted and is presented in blue, simply labeled “Excess of CO
2”. Therefore, the transported mass of CO
2 considered in this work’s discussion is labeled “CO
2”.
In contrast to the case of bilobe tanks, the vertical cylinders configuration needs additional ballast when the pressure is below 35 bar so that the balance of mass remains constant. This is because of the reduced storage volume utilization. The loss of CO
2 mass corresponds with the added ballast, as shown in
Figure 6.
In all of the cases, the higher the pressure, the lower the mass of CO2 that can be transported. This is due to the reduction in density with increasing pressure and temperature. Exclusively regarding the amount of CO2, the best option is bilobe tanks at a pressure of 6 bar. With this option, a mass of 21,318 t can be transported while still meeting the balance of mass limit.
As illustrated in
Figure 7, when focusing solely on the mass of the transported CO
2, it becomes apparent that from the 10–15 bar range, cylindrical tanks are preferable due to the rapid decline in CO
2 storage capacity in bilobe tanks at operating pressures beyond this range. However, a detailed calculation of a bilobe tank may result in a higher decisive pressure range, which may be in the range of 15 to 20 bar. Only considering CO
2 storage pressure, lower pressures prove to be more advantageous, as they allow a higher mass of CO
2.
The relation of CO
2 mass to tank mass and the ratio of tank volume to cargo hold volume are represented in
Figure 8. These two metrics serve as measures of effectiveness in the realms of mass and volume. For the two KPIs, a higher ratio indicates improved efficiency. On the other hand, a small mass ratio suggests that a greater amount of structural mass is being transported relative to the CO
2. Similarly, a small volume proportion indicates that there is a greater amount of unused space in comparison to the utilized space. In this figure, it is observed that higher pressures lower the efficiency. As anticipated, cylindrical tanks make much less efficient use of cargo space compared to bilobe tanks. Nevertheless, bilobe tanks capitalize on this benefit at low pressures, as structural mass grows quickly. For example, at 25 bar, the mass ratio of the bilobe tank is only 1.4. Focusing on these KPIs, CO
2 transportation at lower pressures will likely result in a reduced cost of acquisition per unit mass of transported CO
2 because the ship’s structural mass significantly influences its final cost. The lower ratios obtained from this approach strongly suggest that the base dimensions of a typical LPG ship may not be optimal for CO
2 transport. Therefore, it can be expected that
Figure 2 and
Figure 3 would exhibit a different geometry if a specialized bulk CO
2 carrier were designed.
The final KPI is the one based on EEDI, which is shown in
Figure 9. It considers the mass of CO
2 for both types of tanks as well as the assumptions listed in
Section 2. As the storage pressure rises, the EEDI-based indicator value also increases. As defined in
Section 2, all of the parameters not related to the cargo holds are assumed to remain identical between “Alkaid” and the studied LCO
2 carrier. It can be seen that raising the pressure (and temperature) reduces the CO
2 mass transported on board, thus increasing the EEDI-based indicator. There are also differences between tank types. The slope of the curve for the bilobe tank is much steeper than that of the cylindrical tank. This is because with an increase in pressure, the increase in structural mass is higher in the bilobe tank, as can be observed in
Figure 8a. This illustrates how the ratio between CO
2 mass and structural mass decreases much more rapidly than in cylindrical tanks. In this scenario, a lower value indicates a more efficient ship. This KPI implies that the ship with a 6 bar pressure and a bilobe configuration will consume less fuel per unit distance and unit mass of transported CO
2, likely resulting in a lower freight rate for the low-pressure ship.
From the point of view of CO2 marine transportation, the storage pressure on board can be a relevant factor in the total cost of the CCUS value chain. Transporting CO2 at lower pressures allows for carrying a greater mass of CO2 for both types of tanks and lighter tank structures, which leads to lower shipbuilding costs and higher gains for each voyage. However, lower pressure demands more energy for the liquefaction of CO2, as the temperature is much lower than the ambient. In contrast, higher CO2 pressures lead to the opposite effects. The amount of CO2 transported decreases, and the usage of the cargo hold is worse than with low pressures, but the energy consumed in liquefying the CO2 is reduced.
4. Conclusions
This article aims to determine the most advantageous pressure range for liquid CO2 transportation in vessels by covering the liquefaction of CO2 prior to shipping and its shipment. To this end, this article performs a techno-energetic study through the use of a series of indicators.
The outcomes from the various analyses conducted lead to conflicting conclusions. From an exergetic perspective, liquefying CO2 at higher pressures within the range studied demonstrates superior efficiency than at lower pressures. It can be expected that liquefaction plants aiming for higher pressures of liquefied CO2 might present energy savings compared with their counterparts at lower pressures and temperatures. Furthermore, reducing the CO2 pressure allows for transporting a greater mass of cargo, which likely results in lowered freight rates. Additionally, the decreased structural weight may lead to lower acquisition costs. Given that the entire CO2 logistic chain involves preprocessing (liquefaction), transport, and post-processing (re-gasification) costs, determining the optimal transport pressure is not straightforward. Modifying the CO2 transportation pressure can lead to contrary outcomes for the liquefaction prior to shipping and the shipment itself. Therefore, further investigation is needed to uncover the pressure compensation and the factors that influence it by analyzing the complete CCUS sequence.
It is important to carefully study the design and calculation of the storage tank, as simplified models of bilobe tanks may have excessive uncertainties. There is a CO2 storage pressure beyond which a bilobe tank may not be the optimal choice due to its weight. For higher pressures, the proposed configuration of cylindrical tanks would be a more suitable option, even though they may have lower volumetric efficiency. Additionally, special attention should be given to the transportation of CO2 at lower pressures. Within this range, there is an elevated possibility of inadvertent solid formation of CO2, which has the potential to obstruct pipes or cause damage to pumps.
The authors of this work are currently working on the development of techno-economic models to study the complete logistic chain from CO2 at ambient pressure and temperature, its liquefaction, and its delivery by ship to a CO2 terminal, with the aim of determining an estimate cost per unit mass of transported CO2. The readers should expect a follow-up article from the same authors in the near future.